1. Field of the Invention
The conventional composite fiberboards, typical of those used for acoustical ceilings, are composed of mineral wool, perlite and newsprint, which is primarily cellulosic fibers, as the primary ingredients. These materials are formed into boards from aqueous slurries using an organic binder such as starch.
The mineral wool may be composed of fibers of rock wool or basalt wool. It is also possible to use glass fibers, alone, or mixed with the mineral wool. The fibers, in general, have a diameter of 3 to 6 microns. The fibers may be used in the "sized" or "naked" state. Sizing agents such as mineral oils or acrylic polymer dispersions may be employed. These fibers contribute to the structural integrity and strength of the board.
The perlite is a volcanic glass ore composed of aluminum, calcium or other alkaline earth silicate. Prior to use in the fiberboard process, the perlite ore is expanded at high temperatures to obtain densities of 2 to 10 pounds/cubic foot (pcf), preferably 3 to 7 pcf. Perlite contributes to the bulk and hardness of the board.
The third important ingredient is the so-called "newsprint". Specifically, the newsprint is composed of cellulosic fibers. These fibers contribute to the wet strength of the board as it is converted from the slurry to the substantially solid cake enroute to becoming the board. Hereinafter, these fibers will be referred to as "cellulosic newsprint" fibers.
The mixture may also contain fillers, flame-proofing agents, pigments, water repellants, etc. The fillers employed are usually finely divided clays, e.g. kaolin, ball clay, etc.
ln the process of preparing the board, the ingredients are mixed together with the amount of water necessary to provide slurry consistency in conventional mixing and holding equipment. Additional water and "dry broke" may be added to the slurry prior to the addition of the starch binder. The starch is employed in amounts as high as about 15%, based on the three primary ingredients. The "dry broke" is predominately recycled board material that may have been rejected or cut from the commercially acceptable boards, as well as other waste products.
The slurry is then flowed onto the board forming wire of a Fourdrinier through a conventional head box. Suction may be applied as well as pressure, if desired, to assist in drainage and compaction using conventional means.
The disadvantage of these conventional sound-insulating boards is their moisture sensitivity. Their tendency to sag in a moist atmosphere may make it necessary to coat or impregnate the back and/or face of the boards with, for example, thermosetting plastics or other moisture-resistant compositions. Not only does this add the extra coating step, but further drying and heating becomes necessary. In short, a very expensive board results.
2. Description of the prior Art
ln U.S Pat. No. 4,587,278, sound-insulating boards which are based on mineral fibers and thermoplastic binders are disclosed to overcome the moisture-sensitive disadvantages of the starch-bound board. The binders disclosed in this patent are polymers having glass transition temperatures from 38.degree. to 80.degree. C. These binders may be inadequate for the boards to retain dimensional stability without any substantial sag when exposed for prolonged periods at high temperatures with high humidity.
In European Laid-Open patent application No. 0 266 850 published May 11, 1988, the applicant discloses the use of thermoplastic binders (latex compositions) having glass transition temperatures anywhere from 35.degree. C. to 115.degree. C. The boards disclosed contain newsprint, perlite, mineral wool and clay as well as the latex binder. They are manufactured by incorporating the latex binder into a previously prepared aqueous slurry of the other ingredients. The resulting boards, according to the disclosure, display acceptable strength as determined by measuring the modulus of rupture (MOR) in accordance with ASTM 367-78. The modulii of rupture, as disclosed in the published application, vary from about 140 to slightly above 180 psi. The applicant also discloses an improvement in "dimensional stability" as measured by the composite board's sag resistance. Specifically, by exposing a 1.5.times.6 inch (40 mm.times.150 mm) strip of board to 94.degree. F. (35.degree. C.), and 90% relative humidity for 96 hours While retaining a 330 gram weight at its center, the applicant asserts that he obtained a displacement of the center of the board of 1.0 mm or less.